My last hospital I had to drink 2 pints of water and not empty bladder for hour before - this is to push the uterus up so the ultrasound equipment is closer to the baby. OMG - I thought I was going to explode by the time I got to the hospital and had no choice but to have just a little wee just to relieve a little pressure!
This time, different hospital, it states explicitly you need NOT attend with a full bladder. Also I had a wee scan at 8 weeks and it was very short notice so I had no time to even think about how full my bladder was, and they were able to find baby and zoom in without a problem, though the resolution was quite poor on the printout.
So perhaps it depends on the type of equipment they are using. I would check with the hospital but if they say you don't need an full bladder, then don't put yourself through it hun!
LBxx